Present day personal scales, whether electronic, mechanical, or a combination thereof, require moving parts, such as the displacement of a coil, a moving beam, a rotating wheel wherein the rotational angle thereof is electronically sensed, and the like. Moving parts are subject to wear, which is accelerated when moisture, dust, or other harmful substances contaminate the particular parts or components, resulting oftentimes in inaccurate weight measurements.
The present invention provides a weight sensing device, typically a personal scale, characterized by an absence of movement of any parts. The device employs a thin polymeric piezoelectric film of KYNAR.RTM., for example, as the transducing material, a polyvinylidene fluoride trademark product of Pennwalt Corporation, Philadelphia, Pa. assignee of the present invention. The voltages generated by the piezofilm of polyvinylidene fluoride, upon application thereto of mechanical pressure, i.e., when a weight is applied to the film, are then displayed digitally, in pounds, for example, by electronic means.
The volume resistivity, .rho., of KYNAR.RTM. polyvinylidene fluoride, hereinafter referred to as PVDF, is about 100 to 1,000 times greater than that of a typical ceramic piezoelectric material while the clamped dielectric constant; .epsilon.; of PVDF is about 50 to 100 times smaller. If a charge is developed upon stress of the component equivalent to an open circuit voltage hold condition, the time constant, which determines internal leakage, will be the product of .rho. and .epsilon., which is approximately 1200 seconds for PVDF. This value is about the same or up to about 20 times of longer duration than typical ceramics. The clamped open circuit voltage output for PVDF is about 5 times greater than that of the best ceramics and is thus more sensitive. This greater sensitivity increases the signal-to-noise level very favorably. High .rho..epsilon. time constant of PVDF is most desirable.
Further, ceramic materials are brittle and tend to crack upon application of sudden pressures or concentrated forces placed thereon. It is appreciated that certain powdered polar ceramic materials however may be admixed with a polymer base to provide a flexible piezoelectric polymeric film. These composite materials exhibit less brittleness than pure ceramics.